Violent strombolian and subplinian eruptions at Vesuvius during post-1631 activity

Bull Volcanol (2001) 63:126-150 DOI 10.1007ls004450100130

Simone Arrighi �9 Claudia Principe �9 Mauro Rosi

Violent strombolian and subplinian eruptions at Vesuvius during post- 1631 activity

Received: 12 May 2000 / Accepted: 5 February 2001 / Published online: 9 May 2001 �9 Springer-Verlag 2001

Abstract On the basis of historical chronicles and field investigations the tephrostratigraphic sequence of post- 1631 activity of Vesuvius is reconstructed. It has been established that, during this period, in addition to numerous totally effusive eruptions and/or normal strombotian activity, 16 explosive events produced well-traceable tephra deposits in the area outside the Mount Somma caldera. Ages of tephra beds were established on the basis of stratigraphic relationships with historical lava flows and comparison with chroniclers information. The dispersal and lithological characteristics of tephra deposits combined with description of explosive activity lead to the identification of three styles: (a) periods of violent strombolian activity; (b) violent strombolian eruptions; and (c) subplinian eruptions. Violent strombolian eruptions and periods of discrete activity are characterized by the formation of lapitli falls from eruptive columns only some kilometers high. Subplinian eruptions are defined on the basis of their lapilli fall volumes which is of the order of 107 m 3, on eruptive column heights of approximately 10 km, bt higher than 1.5, and mass discharged rate values not lower than 106 kg/s. During the first century of activity after the 1631 eruption, two periods of violent strombolian activity occurred at Vesuvius

Editorial responsibility: J. Gilbert

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S. Arrighi Dipartimento di Scienze della Terra, Universith di Pisa, Via Santa Maria 53, 56127 Pisa, Italy

C. Principe (~) Istituto di Geoscienze e Georisorse Isotopica CNR Pisa (IGGI), Area della Ricerca CNR di Pisa San Cataldo, Via G. Moruzzi l, 56010 Pisa, Italy e-mail: c.principe @ iggi.pi.cnr.it Tel.: +39-050-3152335, Fax: +39-050-3152360

M. Rosi Dipartimento di Scienze della Terra, Universith di Pisa, Via Santa Maria 53, 56127 Pisa, Italy

(1682-1707 and 1707-1719) preceded, and followed, by a series of violent strombolian eruptions (1660, 1682, 1707, 1723, 1730, 1790, 1872). Between 1730 and 1779 a relevant change in the eruptive style of Vesuvius occurred by an increase in the explosivity of the eruptions. During the past two centuries of activity, only a few eruptions reached subplinian magnitude and only five eruptions had a phreatomagmatic phase (1779, 1794, 1822, 1906, 1944). Therefore, the previously accepted model of cyclic activity, in which each cycle is closed by an important explosive eruption with phreatomagmatic characteristics, is unfounded. The tephrostratigraphy of the 1906 eruption proposed in this work differs substan- tially from some previous reconstructions, on which the basis for the modeling of Vesuvius' behavior in this time span was formed.

Keywords Strombolian eruptions �9 Violent strombolian eruptions. Subplinian eruptions �9 Tephrostratigraphy - Historical chronicles �9 Post-1631 Vesuvius activity. 1906 Vesuvius eruption

Introduction

Somma-Vesuvius is a composite volcanic system com- prising (a) the Vesuvius main cone (1281 m a.s.1.) grown after the famous 79 A.D. plinian eruption, and (b) the pre-existing Mount Somma composite caldera (5 km max. diameter). The floor of the caldera (Fig. 1), surrounding the Vesuvius main cone (Gran Cono Vesuviano), has been partially filled by deposits of the historical activity (79 A.D. to 1944), its lowest elevation being at present approximately 800 m a.s.1. The northern rim of Mount Somma caldera is still a prominent morphological feature which forms an abrupt, crescent- shaped cliff approximately 200-300 m high, reaching, at present, a maximum elevation of 1131 m a.s.1. Other evident morphological features of the Mount Somma caldera floor are the dome-shaped accumulations of the lava flows of Colle Margherita (1891-1894) and Colle

Fig. 1 Location map of measured stratigraphic sections (see Table 1)

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Umberto (1895-1899), and the Atrio del Cavallo-Valle dell'Inferno narrow morphological corridor which ex- tends from west to east tor approximately 4 km and separates the Mount Somma cliff from the northern base of the Gran Cono. Such a clear morphological separation between Mount Somma and Vesuvius is not visible in the southern sector, in that the much lower Mount Somma caldera rim has been almost completely obliter- ated by the post-1631 effusive activity of Vesuvius.

During the period 79 A.D. to 1631 only two major explosive eruptions took place (in 472 A.D. and in 1631), other activity being restricted to minor explosive and effusive events (Andronico et al. 1996, and references therein). After the 1631 eruption, numerous effusive episodes occurred until the last eruption in 1944, and numerous of these were also accompanied by explosive phases (Aru6 et al. 1987) with production of tephra deposits volumetrically smaller than those of the Middle Ages events (Fig. 2a).

The aim of this paper is to present a detailed description of the tephra deposits from the post-1631 period, and to discuss the various eruptive styles which have characterized Vesuvius during its past three centuries of activity, on the basis of physical characteristics and dis- persa[ patterns of its tephra deposits.

Activity of Vesuvius after the 1631 eruption

After the 1631 eruption, Vesuvius volcano experienced a three century-long periods of semipersistent activity. Predominantly effusive eruptions, each separated by an average of 3 4 years (never exceeding 7 years), occurred in this period.

The semipersistent activity of the volcano made Vesuvius the most famous volcano worldwide and the birthplace of modern volcanology. The activity was con- centrated at the summit crater, at subterminal vents and, occasionally, at vents opened at low elevation outside the Mount Somma caldera.

As a result of this activity, Vesuvius experienced al- ternating constructive and destructive phases. The strombolian activity within the summit crater built up scoria cones, which sometimes overpassed the crater rim thus building a single composite main cone.

On the other hand, the summit part of Vesuvius cone was periodically destroyed both during the explosive phases and by collapses which occurred at the end of major eruptions. At the same time, the semicircular de- pression between the Mount Somma caldera and the Ve- suvius cone was partially filled with tephra and repeated lava flows, coming either from the summit crater of Ve- suvius or from eruptive fissures opened at the base or on the flanks of the main cone (e.g., the lava flows of 1855, 1858, 1867, 1872, 1891-94 Colle Margherita, and 1895-99 Colle Umberto).

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Fig. 2 a Stratographic relationships between the tephra emitted from Vesuvius during the past two millennia�9 The erosional gully affecting the 472 A.D. deposits and filled by the Middle Age products testifies to the renewal of the hydrographic network after the 472 A.D. eruption, b Lapilli and ash fall erupted in 1794 fill the evident plough furrows in the underlying agricultural soil (section 39). c Lower portion of the stratigraphic sequence of the post- 1631 tephra, directly overlying the phreatomagmatic ashes of 1631 (section 4). d Erosional unconformities between the 1682, 1779, and 1906 lapilli fall deposits on the Mount Somma rim (section 43); the s h o v e l is 52 cm. e Fine-grained lapilli layer ( C F M I )

of the 1794 eruption underlying the thicker fine gray ash deposit ( C F M 2 ) as exposed at section 5. f Glowing avalanche of the 1822 eruption (section 20). This deposit is interbedded within the lapilli fall sequence. The s h o v e l is 92 cm

Outside the southern rim of Mount Somma caldera, in the SW sector, vents formed at low elevation at least three times: in 1760, 1794, and 1861.

Most post-1631 to 1944 eruptions had a mixed explosive/effusive character. Occasionally, as in the 1779 eruption, activity was almost entirely explosive. The major damage was produced by lava flows, which invaded settled areas and farmland, and tephra falls that sometimes produced severe devastation. Lavas commonly flowed down the seaward-facing slope of the volcano but could not descend the northern flank of Mount Somma

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because of the morphological barrier of the high wall of the caldera.

At present, Vesuvius is in a phase of quiescence which started after the 1944 eruption.

Previous studies

The geological literature on the recent activity of Ve- suvius consists of numerous publications concerning the lavas emitted during the past 350 years. The products of Vesuvian effusive activity during the period post-1631 were mapped by Le Hon (1865), Johnston-Lavis (1891), and by Rosi et al. (1987). In contrast, tephra erupted in this period has received little attention.

Only the two most recent major eruptions, 1906 and 1944, have been studied in some detail (Dolfi and Trigila 1978; Scandone et al. 1986; Bertagnini et al. 1987, 1991; Hazlett et al. 1991; Mastrolorenzo et al. 1993: Pesce and Rolandi 1994; Fulignati et al. 1996).

Despite the lack of relevant studies for the entire period of activity and particularly for its explosive component, Vesuvius activity in the post-1631 period has been repeatedly modelled by means of studies based on historic sources (Carta et al. 1981; Arn6 et al. 1987; Brocchini

129

1995; Principe and Brocchini 1995; Nazzaro 1998), or petrological aspects generally related to magma composition in single eruptions (Cortini and Scandone 1982; Joron et al. 1987; Civetta and Santacroce 1992; Santacroce et al. 1993; Marianelli et al. 1995, 1999; Fulignati et al. 1998).

Tephrostratigraphic sequence

The reconstructed tephrostratigraphic sequence contains the deposits of 16 eruptive events and consists essentially of lapilli and ash fall beds. Sometimes a deposit is composed of a single lapilli package, produced as a result of a sustained eruptive phase. Other deposits origi- nated from several eruptive pulses. Only a few eruptive episodes have produced hot avalanche and lahar depos-

its. At a distance of approximately 3.5 km E from the Vesuvius crater the post-1631 tephra succession reaches its maximum thickness of approximately 4 m. The overall dispersal of the fall tephra package is toward the east.

The lowermost tephra bed of the stratigraphic sequence frequently rests directly on the deposits of the 1631 eruption (Fig. 2c). In the eastern sector of the volcano the 1631 deposits are represented by lapilli fall overlaid by the pyroclastic flow deposits or by fine ash beds intercalated with lahar deposits. In the north-north- eastward sector of the volcano, where the 1631 deposits are lacking, tephra deposits of the post-1631 activity often overlie the pyroclastic flow deposits of the 472 A.D. eruption.

The locations of all measured stratigraphic sections are shown in Fig. 1 and are described in detail in Table 1.

Table 1 Location of the measured post-1631 stratigraphic sections. Geographic latitude and longitude are from 1:5000 topographic map of "Cassa del Mezzogiorno"

Section Longitude E Latitude N Locality name

1 14" 29' 04" 40 ~ 48' 26" 2 14 ~ 28' 25" 40 ~ 48' 37" 3 14 ~ 27' 59" 40 ~ 48' 47" 4 14 ~ 27' 50" 40 ~ 48' 50" 5 14 ~ 28' 30" 40 ~ 48' 52" 6 14 ~ 28' 53" 40 ~ 48' 54" 7 14 ~ 29' 00" 40 ~ 48' 54" 8 14 ~ 29' 09" 40 ~ 48' 58" 9 14 ~ 28' 51" 40 ~ 49' 20"

10 14 ~ 28' 35" 40 ~ 49' 33" 11 14 ~ 28' 17" 40 ~ 49' 28" 12 14 ~ 28' 45" 40 ~ 49' 02" ! 3 14 ~ 29' 07" 40 ~ 49' 02" 14 14 ~ 28' 53" 40 ~ 49' 17" 15 14 ~ 28' 20" 40 ~ 48' 20" 15b 14 ~ 28' 07" 40 ~ 48" 06" 16 14 ~ 28' 01" 40 ~ 48' 36"' 17 14 ~ 28' 48" 40 ~ 48' 40'" 18 14 ~ 28' 40" 40 ~ 47' 42'" 19 14 ~ 28' 24" 40 ~ 50' 15" 20 14 ~ 27' 47" 40 ~ 48' 06" 20b 14 ~ 28' 00" 40 ~ 47' 54" 21 14 ~ 28' 16" 40 ~ 48' 37" 22 14 ~ 28' 18" 40 ~ 48' 04" 23 14 ~, 25' 53" 40 ~ 47' 19" 24 14 ~ 24' 40" 40 ~ 47' 45" 25 14 ~ 27" l 1" 40 ~ 50' 56" 26 14 ~ 26' 06" 40 ~ 50' 35" 27 14 ~ 27' 47" 40 ~ 49' 48" 28 14 ~ 27" 31" 40 ~ 50' 14" 29 14 ~ 23' 08" 40 ~ 48' 31" 30 14 ~ 23' 37" 40 ~ 47' 32" 31 14 ~ 28" 30" 40 ~ 49' 44" 32 14 ~ 29' 11" 40 ~ 50' 24" 33 14 ~ 28' 54" 40 ~ 48' 04"" 34 14 ~ 25' 48" 40 ~ 50' 58" 35 14 ~ 25' 28" 40 ~ 50' 51" 36 14 ~ 27' 50" 40 ~ 48' 13" 37 14 ~ 29' 14" 40 ~ 49" 17" 38 14 ~ 28' 24" 40 ~ 49' 51"" 39 14 ~ 29' 51" 40 ~ 48' 53" 40 14 ~ 30' 32" 40 ~ 48" 35" 41 14 ~ 24' 58" 40 ~ 45' 37" 42 14 ~ 26' 24" 40 ~ 50' 09"

Main road cut near Casale Pacicchi (Terzigno) Excavation near Az. Agricola Fabbricini (E of Terzigno) Small quarry (E of Terzigno) Terracing cut uphill of Terzigno Road cut La Piana Tonda Cut along Campitelli-S.Pietro Town Road (Terzigno) Cut along Campitelli-S.Pietro Town Road (Terzigno) Excavation near Terzigno Main Road Excavation Fossone Small old quarry along rural road to Montagnuolo Excavation near Vallone di Cola Excavation at the end of rural road to Campitelli Road cut Villa Salvati (Campitelli nuovi) Excavation near Fossone Excavation Molara Excavation Molara Cut Pastino Excavation near Az. Agricola Fabbricini (E of Terzigno) Excavation Casa Loggie (near Pozzelle quarry) Scudieri old quarry Pozzelle quarry Pozzelle quarry Cut near Pastino Excavation near Molara (on 1701 lava flow) Excavation La Pagliara on 1760 lava flow (near 1760 vents) Excavation near Casa Sorrentino (on 1804 lava flow) Excavation near It Re della Vigna Excavation Vallone Murello (near Mt. Somma) Excavation Vallone dei Cerri Cut near Ottaviano-Mt. Somma Main Road (near Carcova) Excavation Case Cefariello-Cupa di Spizie (on 1794 lava flow) Excavation near Cappella Bianchini (on 1806 lava flow) Road cut Vicinate-Proficua-Paliata (S.M. La Scala) Road cut Barri Cut near Terzigno Main Road (Caposecchi, on 1730 lava flow) Excavation SE of S. Maria di Castello Excavation Vallone Cancherone (SE of S. Anastasia) Terrioni old quarry Excavation Zabatta Main Road (near Villa Ambrosio) Excavation at the end of vicinale-Palomba road Cut Croce del Carmine Cut Giugliani-Casilli Villa Inglese quarry (near 1760 lava flow) Natural section Cognoli di Ottaviano (Mt. Somma)

130

Table 1 Continued

Section Longitude E Latitude N Locality name

43 14 ~ 26' 37" 40 ~ 49' 57" 44 14 ~ 26' 46" 40 ~ 49' 46" 45 14 ~ 32' 13" 40 ~ 48' 48" 46 14 ~ 31" 52" 40 ~ 50' 20" 47 14 ~ 31' 34" 40 ~ 51' 39" 48 14 ~ 31' 37" 40 ~ 48' 09" 50 14 ~ 24' 44" 40 ~ 48' 41" 51 14 ~ 25' 06" 40 ~ 48' 23" 52 14 ~ 25' 43" 40 ~ 48' 24" 54 14 ~ 26" 11" 40 ~ 48' 27" 55 14 ~ 26' 24" 40 ~ 48" 32" 56 14 ~ 26' 21" 40 ~ 48' 14" 57 14 ~ 25' 36" 40 ~ 47' 36" 59 14 ~ 25' 24" 40 ~ 47' 29" 60 14 ~ 25' 24" 40 ~ 47' 19" 61 14 ~ 25' 26" 40 ~ 47' 02" 62 14 ~ 24' 26" 40 ~ 47' 59" 63 14 ~ 26" 55" 40 ~ 46' 54" 66 14 ~ 26' 22" 40 ~ 46' 40" 67 14 ~ 25" 54" 40 ~ 46' 43" 69 14 ~ 27' 07" 40 ~ 46' 04" ZA 14 ~ 28' 42" 40 ~ 50' 19" ZB 14 ~ 28' 57" 40 ~ 50' 49" ZD 14 ~ 29' 17" 40 ~ 50' 38" ZE 14 ~ 29" 15" 40 ~ 50' 40" ZF 14 ~ 28' 06" 40 ~ 51' 18" ZG 14 ~ 27' 54" 40 ~ 51' 10" ZH 14 ~ 28' 07" 40 ~ 51' 08" ZI 14 ~ 28' 03" 40 ~ 51' 41" ZIb 14 ~ 27' 58" 40 ~ 51' 36" ZIc 14 ~ 28' 07" 40 ~ 51' 34" ZJ 14 ~ 28' 07" 40 ~ 51' 44" ZK 14 ~ 28' 17" 40 ~ 51' 40" ZKb 14 ~ 28' 00" 40 ~ 51' 33" ZL 14 ~ 28' 08" 40 ~ 49' 14" ZM 14 ~ 29' 50" 40 ~ 47' 47" ZN 14 ~ 27' 08" 40 ~ 47' 25" ZNb 14 ~ 27' 07" 40 ~ 47' 18" ZO t4 ~ 26' 44" 40 ~ 47' 15" ZP 14 ~ 28' 46" 40~ 50' 27" ZQ 14 ~ 28' 27" 40 ~ 50' 38" ZR 14 ~ 28' 35" 40 ~ 50' 40" ZS 14 ~ 27' 52" 40 ~ 50' 04" ZT 14 ~ 28' 22" 40 ~ 50" 02" ZU 14 ~ 28' 04" 40 ~ 49' 57" ZV 14 ~ 29' 06" 40~ 50' 33" ZW 14 ~ 28' 56" 40 ~ 49' 26" ZY 14 ~ 28' 07" 40 ~ 51' 32"

Excavation Cognoli di Ottaviano-Levante (Mt. Somma) Excavation Cognoli di Levante (Mt. Somma) Cut Vastola Cut S.Giuseppe Vesuviano Cut S.Gennaro Vesuviano Cut Piano del Principe Excavation near Baracche Forestali Cut S of the crater (on 1822 lava flow) Excavation S of the crater (on 1822 lava flow) Cut SE of the crater (on I822 lava flow) Cut SE of the crater (under 1906 lava flow) Cut SE of the crater (under 1906 lava flow) Excavation near 1760 vents Cut near 1760 vents Cut near 1760 vents Cut near Cappella Vecchia (on 1760 lava flow) Excavation Casa Menicuccio (on 1804-1806 lava flow) Cut near Boscotrecase cemetery Cut Colonne (on 1631 pyroclastic flow) Fossamonaca vent Cut Masseria Bosco del Monaco Cut Boscariello-Palomba Excavation SE of Ottaviano Excavation along via Zabatta (near Carilli) Excavation along via Zabatta (S.Leonardo-Carilli) Cut Vallone S. Severino Excavation Vallone S. Severino Excavation near palazzo Scudieri Cut between Vaccara and Ponte Zennillo (NE of Ottaviano) Cut between Vaccara and Ponte Zennillo (NE of Ottaviano) Cut between Vaccara and Ponte Zennillo (NE of Ottaviano) Excavation between Vaccara and Ponte Zennillo (NE of Ottaviano) Cut between Vaccara and Ponte Zennillo (NE of Ottaviano) Cut between Vaccara and Ponte Zennillo (NE of Ottaviano) Excavation near old quarry S. Pietro Caposecchi quarry (near 1834 lava flow) Excavation Pennino del Mercante Excavation Pennino del Mercante Cut Case Aniello Excavation Boscariello Excavation S of Ottaviano Excavation S of Ottaviano Cut Pescinale Excavation Pescinale Excavation Vallone Mazzamei Excavation Boscariello (near via Zabatta) Cut near Case Pompili Cut between Vaccara and Ponte Zennillo (NE of Ottaviano)

S o m e sec t ions h a v e b e e n i n v e s t i g a t e d and m e a s u r e d in na tura l cuts , bu t fo r the m a j o r i t y o f t h e m it was n e c e s - sary to d ig t renches . T h e t eph ros t r a t i g r aph i c co r r e l a t i ons b e t w e e n s o m e o f the m o s t r e p r e s e n t a t i v e sec t ions are s h o w n in Fig. 3. T h e sma l l d i s t ances b e t w e e n the inves t i - ga t ed sec t ions a l l o w e d us to t r ace e a c h s ing le t eph ra bed wi th ful l c o n f i d e n c e , thus p e r m i t t i n g f i rm co r r e l a t i ons b e t w e e n the va r i ous layers .

T h e t ephra d e p o s i t s h a v e fa i r ly l imi t ed d i spersa l s , a re o f t en c h a r a c t e r i z e d by s u d d e n th i cknes s changes , and are f o u n d g e n e r a l l y up to a d i s t ance o f not m o r e than 5 k m f r o m the crater. O n the f l oo r o f M o u n t S o m m a c a l d e r a these depos i t s are b u r i e d unde r l a v a f l ows and the t eph ra o f the last (1944) e rup t ion .

T h e shor t t i m e span du r ing w h i c h the t ephra s e q u e n c e was d e p o s i t e d has p r e v e n t e d the f o r m a t i o n o f th ick soi ls . T h e shor t b reaks b e t w e e n e a c h e r u p t i o n are, h o w e v e r , o f ten r e c o g n i z a b l e b e c a u s e o f e v i d e n c e s for p l o u g h i n g (Fig. 2b). It has in fac t to be r eca l l ed that all the a rea a r o u n d Vesuv ius , p r io r to the u n c o n t r o l l e d and w i d e -

Fig. 3 Location map and stratigraphic correlation of five repre- ~" sentative sections of the post-1631 tephra sequence. 1 Present soil; 2 reworked soil; 3 ash; 4 coarse ash; 5 scoria fall with cineritic matrix; 6 pheatomagmatic ashes with silty, pisolitic, and vesiculated tuff beds; 7 fine-grained lapill fall grading to coarse ash; 8 reworked layer with cineritic lenses; 9 lahar; 10 scoria fall with ash coating and abundant lithic fraction; 11 scoria fall

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Table 2 Stratigraphic relations between lava flows of known age and post-1631 tephra. Labels of tephra unit are the same of Fig. 4a. Numbers of stratigraphic sections are in parentheses

Underlying lava flow Tephra layer Overlying lava flow

1817 (36) G/BSR 1834 (ZM) T2 1817 (36)

1760 (23) CFM 1730 (33) TI 1754 (20) 1718-1719 (22) FdL 1730 (33)

spread urbanization of the past 50 years, was essentially agricultural land. On the northern and western sectors of the volcano agricultural activities have been carried out up to 650 m a.s.1, on the slopes of Mount Somma, and they are still in progress, often completely reworking the post- 1631 tephra deposits.

Age definition

The ages of post-1631 lava flows of Vesuvius are known, thanks to accurate descriptions and (especially for the younger ones) mapping in contemporaneous chronicles (Principe et al. 1987). In the SSE sector of the volcano numerous tephra deposits are found interlayered with lava flows of known age. thus constraining their ages in relatively narrow intervals.

For instance, as shown in Table 2, the age range of BSR tephra is constrained between 1817 and 1834 lavas; T2 is older than 1817; CFM is younger than 1760; TI lies between 1730 and 1754, whereas FdL is between 1718-1719 and 1730. On this basis, the reconstructed stratigraphic column (where each tephra layer has been labeled) has been subdivided into numerous age intervals (Fig. 4a).

The eruptive phenomena of the post-1631 period have been described in contemporary chronicles (see Table 3). All historical data related to the presence, distribution, and physical characteristics of the tephra emitted during the explosive phases, as summarized in Table 3, appear to be reliable because they are directly related to the im- pacts they produced on the villages and farmlands of the Vesuvius area. The actual age of each tephra layer (Fig. 4b) has been therefore assigned by comparing the information contained in the contemporary chronicles with the physical and dispersal characteristics of the in-

vestigated tephra beds, which are described in detail herein. It has to be pointed out that among the numerous events listed in Table 3 and described using the terminology of the contemporary accounts, only the deposits of those indicated with an age shown in bold type, as well as those between 1682 and 1719, are actually preserved and are described in this paper. The tephra deposits of all other listed events have to be very minor layers that have been either not preserved, or are buried inside the Mount Somma caldera by lava flows.

Age uncertainty remains only for Tc and T2 tephra layers which could have been emitted either during the 1806 or 1810 or 1817 eruptions.

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qrg Fig. 4a,b Age attribution of po'+t-1631 tephra, a On the basis of stratigraphic relations with lava tlows of known age (see Table 2), the reconstructed stratigraphic column (where each tephra layer has been labeled) is subdivided into numerous age intervals, b The analysis of the historical sources has allowed us to obtain information on the kind of deposits and often about their areal distribution (see Table 3). The comparison between the historical and stratigraphic data has allowed age determination for almost all sur- veyed tephra

Tephra description

Nineteen representative samples were analyzed in the labo- ratory for mineralogy, grain size, and componentry. Each layer of the reconstructed tephra sequence shows distinc- tive characteristics (Fig. 5; Table 4) such as structure and mineralogy, which, in combination with thickness and clast shape, allow reliable correlation all around the volcano.

The 1660 eruption (SO 1 )

The lowermost tephra bed of the reconstructed stratigraphic sequence is a massive black ash fall deposit directly overlying the phreatomagmatic ash of the final phase of the 1631 eruption (Fig. 2c). The contact be-

Table 3 Data on the explosive activity at Vesuvius from 163t to 1944, from historical sources. In bold the age attributed in this paper. The numbers in parentheses refer to historical chronicles. For complete bibliographical quotation and information on the location of these chronicles into some of the major Italian libraries see

133

on the "Library" page of the CNR-Pisa home page (http://bib- lio.area.pi.cnr.it/) clicking on "Accesso integrato ai cataloghi CNR-Pisa", on the data base BIBV: Bibliography of Historic Ac- tivity on Italian Volcanoes

Age Type of explosive activity and deposits Dispersal sector

1660 (SO1)(3-21-23-28-55-71-83) 1660-1682 ~3-21-23-28-55-71-83) 1682 (SO2)(23-33-55-71-83) 1685 (4-18-33-55-71-83) 1689 (4-19-33-55-71-83) 1689-91 (83) 1694 (4-19-23-28-33-55-66-71-83)

1696-1697 (20-33-55-83) 1698 (20-28-33-55-83) 1701(23-33-55-83)

1704-1706 (23-33-55-83) 1707 (SAD1) (23-28-33-55-83)

1708-1712(23-33-55-83) 1714(23-33-55-83)

1717 (16-23-28-33-55-83) 1720-1721 (23-33-55-83) 1723 (FdL)(23-28-33-55-83)

1724(23-28-33-55-83) 1726(23-28-33-55-83) 1727-1729 (23-33-55-83) 1730(T1)(23-28-33-55-83)

1732-34(23-33-83) 1737 (2-24-30-33-80) 1742-43 (55) 1751(23-32-33-55) 1752-54(33-55) 1754 (33-57) 1760(26-32) 1766 (26-32) 1766-67 (26-32) 1767 (26-33-44) 1770-1771(26-33--44) 1771-1776 (26-33-44) 1779(OTV) (2-6-12-25-33-44-85-89)

1783-1790 (26-86) 1790 (Tb)(86)

1791-1793 (87) 1794 (CFM) (7-8-9-14-17-22-40-65-76-79-87)

1798-1822(41-64)

1822 (G/BSR) (11-37-54-64)

1826-1834(48-74)

Dark ash emission (30 days); mud flows Weak strombolian activity Vigorous lava fountain (paroxysm of 8 days); lapilli Weak lava fountains; ashes Weak lava fountains Weak strombolian activity Strombolian activity; ashes

Strombolian activity; ashes Strong strombolian activity; lapilli and coarse dark ashes Strombolian activity (a lava fountain for 11 days); lapilli and ash Weak strombolian activity; dark-gray ash Lava fountains (4 in 16 days); lapilli and ashes; mud flows

Weak strombolian activity; ash Strong strombolian activity (2 lava fountains of 3.5 h); lapilli and ash Lava fountain; black ash Weak semipersistent strombolian activity; ash 18 Strong lava fountains (15 continuous and 3 pulsating, for 106 h in total) Strombolian activity; ash Strombolian activity; lapilli and ash Weak strombolian activity; reddish ash Vigorous lava fountains (29 days in total); lapilli and ash

E (as well as S and NE)

ENE

NW and NE (as far as Naples and Benevento) Naples S, N, and W NE

S ESE (lapilli); N and SW (ashes)

N (Somma V, S. Anastasia, Ottaviano) SW (Torte del Greco, Naples) NE (Ottaviano) E

S SO NE/E/SE NE/E/SE (between Ottaviano and Boscotrecase)

Strombolian activity (only intracrater) Lava tountain (4 days); dark ashes (black to gray) and lapilli N-NE Weak strombolian intracrater activity Strombolian activity; ashes W Weak strombolian activity (only intracrater) Strombolian intracrater activity Strombolian activity; lapilli and black ash Weak strombolian activity; lapilli and ash Weak strombolian activity; lapilli Strombolian activity; coarse ash Weak strombolian activity; lapilli and ash Strombolian intracrater activity Strong lava fountains (two for 7 h) and an explosive phase that is likely to have been a steady column phase (45 min and a maximum column height estimated at 2.7 km), ash emissions (13 h): lapilli layers (the thickest one contains lava lithics sometimes with a rind of fresh lava), gray "wet" ashes (scoria with ash coating), mud flows, hot avalanches Weak strombolian intracrater activity Strong strombolian activity; black ash and lapilli NNE (cylindrical black sustained column) Weak strombolian activity Complex eruption consisted of lava fountains and strong (pulsating clouds) ash emission (13 days); lapilli and particularly gray "wet" ashes, devastating mud flows Weak strombolian activity; lapilli and ash particularly during 1806, 1810, and 1817 events

Polyphase powerful eruption; lava fountain (5 h, H=6-700 m), steady column phase (3 h), strong ashes emission (19 days); lapilli layers (lava lithics inside deposits), gray-reddish "wet" ashes, mud flows, hot avalanches Weak strombolian activity (only intracrater)

Intracaldera area lntracaldera area Intracaldera area W N

Narrow dispersal sector: NE (Ottaviano); ashes arrived as far as the Adriatic sea; lapilli as far as Benevento

All the sector between Somma V. (N) and T. del Greco (SW)

Several localities as Sorrento (S), Nola (NE), Naples (W), Ischia island (WSW), etc. SW (lapilli fallouts), mainly toward NNE for ashes

134

Table 3 Continued

Age Type of explosive activity and deposits Dispersal sector

1837-1839 (48-75) 1839 (48-75)

1841-1850 (43-48) 1856-1858 (48-67) 1861 (48-67) 1865-1866 (48-68) 1867-1868 (48-68) 1871-1872 (48-69) 1872 (T3) (69-70)

1877-1885 (48-81) 1894--1906 (48-59-62--63-82) 1906 ( 13-15-34-35-36--45-47--49- 52-53-60-61-73-77-82-88)

1913-1925 (48) 1929 (46) 1932-1933 (48) 1938-1939 (48) 1941-1943 (48) 1944 (46, 48)

Weak strombolian activity (only intracrater) Strong strombolian activity; lapilli

Moderate strombolian activity; lapilli Weak strombolian activity; only a few ashes Moderate strombolian activity; ash Weak strombolian intracrater activity Moderate strombolian activity; lapilli and black ash Weak strombolian intracrater activity Strombolian activity; lava fountain (3 days for the main phase); scoria, lapilti, ash, and mud flows Strombolian intracrater activity Strombolian intracalder activity; lapilli and ash Polyphase strong eruption; lava fountain (3 h), steady column phase (5 h and Hmax=4.3 m), strong ash emission (121 h); lapilli layers (the thickest one composed of reddish scoria and reddish lava lithics), gray-reddish "wet" ashes, mud flows, hot avalanches Weak strombolian activity (only intracrater) Strombolian activity; scoria emission (3 lava fountains) Weak strombolian intracrater activity Weak strombolian intracrater activity Weak strombolian intracrater activity Polyphase eruption; 8 lava fountains (8 h and 31 min in total), steady column of ashes (H=5-6 km); lapilli, dark ashes, hot avalanches

Southern sector, toward Boscotrecase, Castellammare, and Sorrento Intracaldera area Intracaldera area Intracaldera area

Intracalderaarea

ENE

Intracaldera area NE (Ottaviano, S. Giuseppe V.); ashes arrived as far as Nola and Andria (next to the Adriatic coast)

ESE and SE (mainly Terzigno, S. Giuseppe V. and Poggiomarino)

1 AA.VV, 1685; 2 AA.VV. 1779; 3 Anonymous, 1661; 4 Anony- mous, 1694; 5 Anonymous, 1765; 6 Anonymous (Abate Galiani), i780; 7 Anonymous, 1794 ("Retazione ragionata..."); 8 Anony- mous, 1794 ("L'eruzione del Vesuvio..."); 9 Anonymous, 1794 ("Lettera ragionata..."); 10 Anonymous, 1794 ("Avviso al pub- blico..."); 11 Anonymous, [822; 12 Attumonelli, 1779; 13 Barana, 1906; 14 Barba, 1794; 15 Bassani and Galdieri, 1906; 16 Berke- ley, 1717; 17 Breislak and Winspeare, 1794; 18 Bulifon, 1685; 19 Bulifon, 1694; 20 Bulifon, 1701; 21 Cala', 1661; 22 Caneva (Eremita del SS. Salvatore), 1794; 23 Cavalli, 1769; 24 D'Amato, 1756; 25 De Bottis, 1779; 26 De Bottis, 1786; 27 De Fiore, 1913; 28 De Fiore, 1916; 29 De Fiore, 1923; 30 De Geronimo, 1737; 31 Della Torre, 1751; 32 Della Torre, 1768; 33 Della Torre, 1779; 34 De Lorenzo, 1906; 35 De Luise, 1907; 36 De Luise, 1914; 37 De Nobili, 1822; 38 Di Leo, 1779; 39 Elisei, 1645; 40 Filoma- rino (Duca della Torre), 1794; 41 Filomarino (Duca della Torre), 1804; 42 Giannone (Giano Perentino), 1718" 43 Guarini et al.

1855; 44 Hamilton, 1779; 45 Johnston-Lavis, 1909; 46 Imbb, 1951; 47 Imbb et al. 1959; 48 Imbb, 1984; 49 Lacroix, 1906; 50 Macrino, 1693; 51 Malone, I703; 52 Maltadra, 1926: 53 Matteucci et al., 1906; 54 Mauri, 1823; 55 Mccatti, 1752; 56 Mecatti, 1754 ("Osservazioni..."); 57 Mecani, 1754 ("Descri- zione della lava..."); 58 Mecatti, 1754 ("Discorsi storico- filosofici..."); 59 Mercalli, 1906 ("Notizie Vesuviane..."): 60 Mercalli, 1906 ("L!eruzione Vesuviana..."); 61 Mercal[i, 1906 ("La grande eruzione..."); 62 Mercalli, 1907; 53 Mercalli, 1909:64 Monticelli and Covelli, 1823; 65 Olivieri, 1794:66 Pacichelli, 1695; 67 Palmiefi, 1862; 68 Palmieri, 1870:69 Palmieri, 1872; 70 Palmieri, 1873; 71 Paragallo, 1705; 72 Perillo, 1755; 73 Perret, 1924; 74 Pilla, 1834; 75 Pilla, 1839; 76 Pitaro, 1794; 77 Sabatini, 1906; 78 Sanfelice, 1726; 79 Santoli, 1795; 80 Serao, 1738; 81 Sicardi, 1974; 82 Sicardi, 1975; 83 Sorrentino, 1734; 84 Stoppa, 1806; 85 Tata, 1779; 86 Tata, 1790; 87 Tata, 1794, 88 Toniolo, 1906; 89 Torcia, 1779; 90 Vetrani, 1780

tween these two ash deposits is characterized by a wavy surface at centimeter scale, without sharp boundaries.

The ash bed has a wide distribution, with a greater thickness on the eastern sector which, according to the historical descriptions (reported in Mecatti 1754 "Osservazioni..." and De Fiore 1923; see Table 3), was the most affected. Some thickness values, measured immediately after the end of the eruption, are in full agreement with our new data.

The 1682 eruption (SO2)

The deposits of the 1682 eruption consist of a very thinly bedded scoriaceous lapilli fall, with alternation of coarse- and fine-grained lapilli beds. Coarse lapitli (rare-

ly • >3.5 cm) are often flat and prolate, showing a thin ochre coating of alteration products, whereas fine- grained lapilli (rarely Q >0.5 cm) are black, more equidimensional, and denser.

This bedding is more evident in the sections near the NE-trending dispersal axis (box 1 in Fig. 6a). Away from the dispersal axis, the deposit becomes a single ochre- coated scoria layer only a few centimeters thick (Fig. 2c), rich in loose dark mica crystals (Q <2 ram). Loose crystals of feldspar and pyroxene are also present, with diameters that rarely exceed 2 mm. Lithic fragments are relatively scarce (7.4 wt.%) in the coarse lapilli beds (Table 4) and are represented by millimeter- size red clasts (rarely ~ >0.5 cm).

The deposit of the 1682 eruption reaches a thickness of 30 cm in a complete exposure located approximaa~ly

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Fig. 5 Mineralogy of the juvenile fractions of selected samples of the post- 1631 tephra sequence.

3.5 km NE from the crater (section 32) while it attains, despite evident signs of erosion, a thickness of 37 cm in an outcrop located on the crest of Mount Somma, 1.5 km NE from the crater (Fig. 2d).

The 1682-1707 activity (SO3)

Between 1682 and 1707 almost persistent explosive activity is described as "mainly strombolian" in the historical chronicles (Table 3). Products which, in this study, are attributed to this period consist of black lapilli falls grading upward to coarse ash (Fig. 2c). There are some discontinuous and poorly defined gray ash layers interbedded in this sequence which covers the eastern flank of the volcano. It is 40 cm thick at a distance of approximately 3.5 km ENE from the crater (section 11).

and leucite are also present, whereas lithic clasts are absent (Table 4). The dispersal axis of the 1707 lap�9 fall is toward ESE (box 2 in Fig, 6a). The deposit is massive and has a thickness of only 6 cm at approximately 3.5 km E from the crater (sections 16 and 21). The field measurements agree with those made immediately after the eruption and reported in some historic documents (Sorrentino 1734; Mecatti 1754 "Osservazioni..."; see Table 3).

The 1707-1719 activity (SAD2)

The deposit ascribed to 1707-1719 is a massive, black, coarse ash fall (Fig. 2c), distributed on the eastern sector of the volcano and reaching a thickness of approximately 22 cm at approximately 3.5 km E from the crater (section 11). The historical chronicles (Table 3) described the activity that produced this deposit as "weak and strong" strombolian.

The 1723 eruption (FdL)

The deposits of this eruption consist of scoriaceous lapill�9 and ash fall spread over the eastern flank of the volcano (box 3 in Fig. 6a). The lap�9 layer is very thifaty bedded (Fig. 2c) and consists of an alternation of coarse lapilli beds and finer-grained (Q <0.5 mm) ones (at most eight coarse beds and nine fine ones, plus one generally thicker coarse layer at the bottom of the deposit). The lithic content is very low (1.2 wt.% in Table 4), and millimeter-size (O <2 mm) loose crystals of pyroxene, olivine, dark mica, and feldspar are abundant. The well- defined stratification of the fall deposit fully matches the historic description of this activity (Table 3). It continues to be recognizable moving away from the dispersal axis or proceeding toward the distal sector. The deposit reaches a thickness of 40 cm at approximately 3.3 km E from the crater (section 4).

The 1730 eruption (TI)

The 1730 deposit consists of a thin massive and poorly sorted (~,=2.18) scoriaceous lithic-free lapilli fall (Table 4). Juvenile clasts are highly vesiculated with a fibrous fabric.

This deposit is 7 cm thick at approximately 3.3 km from the crater (section 4) and is mostly dispersed to the east toward Terzigno (box 4 in Fig. 6a).

The 1707 eruption (SADI)

The fall deposit of the 1707 eruption consists of scoriaceous lapilli with a characteristic silvery shine and the presence of very abundant leucite phenocrysts often with glomeroporphyritic textures. Loose crystals of pyroxene

Fig. 6 a Cumulative isopach maps for the lap�9 fall of six erup- 1~ tions (contour values in centimeters). Dots with white ring mark sections relative to each deposit. Black numbers in 1822 and 1906 cumulative isopach maps refer to historically described thicknesses. b Isopach maps for the main layer of the 1822 and 1906 eruptions. For the 1822 ash layer only the dispersal sector is given. Black numbers refer to our thickness data

1 3 8

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1779 ERUPTION [

a

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~ 7 - G r a y ash fall, sometimes with pink ---~. : i .-~ I J shading. �9 ~ , j -

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/ g r a d e d fall, with top. generally finer, =".~,-..'~--~'-.~ / c o m n o s e d of d e n s e and I ' , . C~" . ' ~ , * ' 7 / r- , o~ o "o "~3~ /equ id imens iona l scoria with reddish ash 20 O O '~,i / c o a t i n g (E~max = 3 cm). This layer is well ~ ( ~ k ~ - / s o r t e d and rich in lithic clasts, mainly ~ ' .~' ,~,~ ~ ~ consisting of small vesiculated red ones ~,q �9 ~ / ~ ~k and fresh lavas (ML = 2 cm).

~ Nongraded coarse ash fall lithologically ~ ~ ~ very similarto o'rv2 layer.

~*, . ' ~ , , ~ ---------Nongraded lapilli fall (~rnax = 0.5 cm) QI~'~0 O o~o0 ~ lithologically similar to B layer.

I ~ ,G~u~ (~ ~ N o n g r a d e d fall composed of black glassy and vesiculated scoria (E~max = 1.5 cm). Only a few red vesiculated lithics (size notgreater than 0.5cm).

The 1779 eruption (OTV)

The 1779 deposit is the first one to show a certain degree of stratigraphic complexity that reflects a more complex eruptive mechanism than the eruptions described previously.

The deposit is composed of two main thick lapilli layers (OTV 1 and OTV2 in Fig. 7). OTVI layer consists of highly vesicular black scoriaceous lapilli and is characterized by a low lithic content (1.3 wt.% in Table 4). The overlying OTV2 layer shows a marked color contrast with the black scoria of the underlying basal layer. This effect is due to a reddish ash coating on the lithic and juvenile fractions. OTV2 deposit contains pyroxene, olivine, leucite, and dark mica loose crystals, the latter two less than 2 mm in length (Table 4). Lithic content is significant (25.0 wt.%) and sometimes they consist of lava fragments with a rind of juvenile scoriaceous magma as also described in historical chronicles (Table 3).

The total 1779 sequence reaches a thickness of 21 cm at a distance of approximately 5.5 km NE from the crater, whereas in the most proximal sections (42-44) dug on top of Mount Somma at 1.5 km NE from the crater, it is up to 162 cm thick, even if the exposure is incomplete due to erosion.

The 1779 deposit is poorly preserved due to its narrow dispersal (Fig. 8) and because the land around Ottaviano town has been under intense farming since the seventeenth century. In Fig. 8 some deposit thickness measured by contemporary authors are reported (De Bottis 1779; Della Torre 1779; see Table 3).

The strong dispersal to the northeast, coupled with good sorting (c,=0.96 for OTV1 and 0.93 for OTV2), suggests the presence of a strong southwesterly wind at the time of the eruption as confirmed by historical chronicles. It has been possible to estimate a wind velocity (for OTV2 layer) of 21.5 m/s on the basis of the onset of darkness in the various localities progressively reached

Fig. 8 Dispersal sector of the 1779 eruption fall deposits. Black bold numbers refer to the thicknesses (in centimeters) given by the historical chronicles

by the eruption cloud, as given by contemporary chronicles (see Table 3; Torcia 1779).

The 1794 eruption (CFM)

Two main layers are recognized in the 1794 eruption (Fig. 2e): The basal one (CFMI) consists of a fine-

t40

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I - ~ i . . 1 , , "~l . -~" F on0ra0e0,a0,,.*a,, = 0 o s e 0 o*~ re . . , . . ~ _ ~ coated scoria and rich in lithic clasts.

~ ---"------- Reddish coarse grained ash fall.

~, " ~ ~ ~ Symmetrically (reverse to normal) graded lapilli fall ~ - ' / ~ ~ composed of dark scoria with reddish ash coating, ~ ~ - - l i t h o l o g i c a l l y and mineralogically very similar to o ,~, o O , layer G1 (~max = 5 cm, ML = 3 cm).

* ,D Q ,

z ." p O _ * 0 r

.-.ooo.,.,

~ 02~

~ R e d d i s h coarse grained ash fall (clasts are generally < 0.5 cm).

S Normal graded lapilli fall composed of dark scoria (Omax =3 cm) with reddish ash coating, denser and more porphyritic than GO layer scoria. This layer is rich in lithic clasts, mainly consisting of red vesiculated ones and fresh lavas sometimes with a juvenile rind (ML = 2 cm).

- - Nongraded lapilli fall composed of dark scoria with reddish ash coating.

Weakly reverse graded lapilli fall composed of black vesiculated scoria (~3max = 5 cm).

Fig. 9 Type section (section 20) of the 1822 eruption. The main macroscopic physical characteristics of fall layers are described

grained fall of equidimensional scoriaceous lapilli ( 0 <0.5 cm), 4-5 cm thick and containing millimeter- size red lithics (12.0 wt.%). Only in one exposure (section 20) CFM1 overlie a very thinly bedded layer (CFMIa) composed of juvenile fragments, represented by highly vesiculated and stretched black lapilli with a high content of crystals (38.7 wt.% in Table 4). The upper fall layer (CFM2) consists of gray fine ash sometimes very thinly bedded. In a few exposures pink beds and coarser ash beds with subordinate millimeter-size red lithics are visible. The presence of accretionary lapilli and vesiculated tuff beds suggests the presence of abundant moisture in the eruptive cloud. The 1794 ash deposit occupies a wide sector to the east of the volcano with a fairly uniform thickness and reaching a maximum thickness of 40 cm at approximately 4 km E from the crater (section 36). The few thickness values found in historical chronicles (Filomarino 1794; Tata 1794, see Table 3) are in good agreement with the thicknesses measured during the present field work in 36 sections.

The uppermost part of the sequence of the 1794 eruption is, at a few locations, composed of lahar deposits with an abundant ash matrix, corresponding to the devastating mud flows described in historical chronicles (Ta- ble 3). The one at section 32, approximately 3.5 km NE from the crater, is composed of ten distinct flow units, is 1.8 m thick, and overlies the primary ash deposit. In fact,

the deposition of primary fine ash leads to the formation of an impermeable blanket that prevents the rain water from being absorbed by the underlying soil, thus sub- stantially increasing the runoff and triggering the formation of lahars as stated for all other Vesuvian eruptions with an important phreatomagmatic phase, like the 1631 one (Rosi et al. 1993).

Minor tephra layers (Tb, Tc, T2, T3)

Four lapilli falls with characteristics similar to each other are included in the tephra sequence. They have been attributed (Fig. 4) to 1790 (Tb), 1872 (T3), and to two eruptions belonging either to 1806 or 1810 or 1817 events (Tc and T2).

They are all characterized by a constant low thickness (max. 12-13 cm) outside the caldera rim. The deposits consist of highly vesiculated black scoriaceous lapilli with a fibrous fabric (similar to the 1730 scoria) with a significant content of millimeter-size (6 <2 mm) leucite as loose crystals and the nearly total absence of lithic clasts (Table 4).

The 1822 eruption (G/BSR)

The deposits of the 1822 eruption consist, from base to top, of: (a) lapilli fall; (b) ash fall; (c) hot avalanche; and (d) lahar.

The 1822 fall deposit (Fig. 9) consists of three main scoriaceous lapilli layers (GO, G1, G2), interbedded with thinner scoriaceous lapilli beds always visible in sections located near the dispersal axis (BSR). The basal layer GO is virtually lithic-free (3.2 wt.%), whereas the two other layers, G1 and G2, are very similar to each other, characterized by the presence of an abundant lithic fraction (18.6 and 23.3 wt.%, respectively; see Table 4) and by a reddish ash coating of the clasts. The thin layer which separates G1 from G2 (BSR2) is finer grained (0 <0.5 cm), and although with a maximum thickness of only 2.5 cm, is always present in all investigated sections.

The dispersal sector of these deposits is shown in box 5 of Fig. 6a in which cumulative isopachs of lapilli fall have been drawn including the thickness values found in historical chronicles (Monticelli and Covelli 1823; see Table 3). The historical values are in good agreement with the data collected during the present field work in 34 measured sections. The fall deposits of this eruption are the only tephra of post- 1631 activity preserved on the SSW flank of Vesuvius.

The uppermost part of the 1822 sequence is composed of an ash fall (BSR4) deposit that shows a wider distribution than the lapilli fall sequence, being present also in some exposures on the NE sector of Vesuvius. This deposit reaches a maximum measured thickness of approximately 25 cm at a distance of approximately 4 km ESE from the crater (section 20). The isopach maps related to the different layers of the 1822 sequence are given in Fig. 6b.

The layers overlying the basal GO (box 2, Fig. 6b) are dispersed more to the east, compared with GO (box 1, Fig. 6b), although they maintain a general southeast dispersal. The top layer of BSR4 ashes is dispersed mainly to the northwest (box 3, Fig. 6b) and can be attributed, on the basis of historical chronicles, to the 24-27 October 1822 activity. These data indicate either an anticlockwise shift in wind direction during the eruption or different wind direc- tions corresponding to different eruptive column heights.

Lahar deposits, overlying the ash layer, have been found at a few locations. They are hyperconcentrated flow deposits poor in fine matrix and occur as units less than 1 m in thickness.

In only one locality (ca. 4 km SE from the crater, section 20) a hot avalanche deposit was also recognized (Fig. 2f). This matrix-supported deposit is non-graded, poorly sorted ((y,=3.32; Md~=-l.77), and composed of a coarse ash matrix with lithic and juvenile blocks up to 50 cm in diameter. The lithic content is 53.7 wt.%, whereas juvenile fraction is 45.4 wt.% (Table 4, sample AM51). The emplacement of this hot avalanche deposit has been described by Monticelli and Covelli (1823; see Table 3) in the place effectively corresponding to section 20 of this work.

The 1906 eruption

The deposits of the 1906 eruption consist, from base to top, of: (a) lapilli fall; (b) ash fall; and (c) lahar. The de-

141

posits lie in the northeastern sector of Vesuvius, with a dispersal axis confined to an area between Ottaviano and San Giuseppe Vesuviano (box 6, Fig. 6a). The fall deposit is composed of four main scoriaceous lapilli layers (Exl, Ex2, Ex3, Ex4) interbedded with finer-grained thin scoriaceous lapilli fall beds (S 1, $2, $3) always visible in the sections located near the dispersal axis (box 4, 5, 6; Fig. 6b). The complete fall sequence is visible in the type section located on the Mount Somma crest, at approximately 1.5 km NE from thc crater (Fig. 10).

Exl layer has a very low lithic fraction (0.3 wt.%), represented by millimeter-size oxidized clasts. In contrast, the overlying layers are characterized by higher lithic contents (Table 4) and, except for Ex2, by a reddish ash coating of clasts. Ex3 is the thickest layer of the sequence and shows a marked color contrast with the underlying layers of black scoria (Fig. 10). The loose crys- tal fraction is represented by up to centimeter-size pyroxene and olivine, whereas dark mica, leucite, and feldspar are generally smaller than 2 mm (Table 4). This deposit has a high lithic content (63 wt.%) and is relatively well sorted (cro=l.90). In the most complete sections near the dispersal axis the upper part of this layer shows some bands 2-3 cm thick with a slight granulometric change and a more or less marked reddish ash coating of the clasts, described also in the stratigraphy of Johnston- Lavis (1909). Ex4 has a lithic content of 61 wt.%, is well sorted ((~,=1.48), and scoria are poorly vesicular and more equidimensional compared to Ex3. Loose crystals of pyroxene can exceed 0.5 cm in length, whereas dark mica and feldspar are millimeter sized. In some exposures located at approximately 5.5 km NE from the crater hyperconcentrated flow deposits with scarce fine matrix overlie the top of the primary fall sequence.

The isopachs shown in box 6 of Fig. 6a refer to the cumulative scoria fall and have been drawn taking into account the thicknesses measured by contemporary authors (see Table 3) which are in good agreement with 47 field measurements carried out during the present study. They have contributed to draw accurate isopach map, especially taking into account that the upper part of the 1906 deposit has often been reworked by anthropic activity.

From the historical data a wind velocity of 16 m/s has been estimated for Ex3 phase on the basis of the progressively darkness caused by the eruptive plume (Imb6 et al. 1959; see Table 3).

Previous stratigraphic reconstructions of the 1906 eruption

Previous tephrostratigraphic studies of the 1906 eruption include those of Johnston-Lavis, Bertagnini et al. (1987, 1991), and Mastrolorenzo et al. (1993). The stratigraphic sequence reconstructed in the present work is in agreement with Mastrolorenzo et al. (1993). The description of the 1906 deposits made by Johnston-Lavis immediately after the eruption coincides perfectly with our field

142

Fig. 10 Type section (no. 42) of the 1906 eruption with the description of macroscopic physical characters of fall layers

1906 ERUPTION ] Cm

160

140

120

100

80

60

40

20

0

r ~ - - - ( d ~ - -%

_ '2o2o, _,

c J r O

M.,.) ~ Q,.n"

_

- - ~ , - " 7 �9 (

- ~ ~ o

UJ

( - 7

f Gray-pink fine-grained ash fall (grey coloured at the top and pink coloured at the bottom), sometimes overlaid by lahar deposits.

} ~ S c o d a lapilti and ash fall (~rnax = 2 cm).

___.---------Symmetrically graded (normal to reverse) thinly bedded lapilli fall composed of reddish ash coated scoria. This layer is similar to Ex4 level but it can be divided into 3 different beds

- ~ on size and colour (the characteristics middle

~ bed is the finest).

Nongraded lapilli fall composed of reddish ash coated scoria denser than Ex3 layer. Lithic clasts are very abundant, mainly consisting of fresh lavas sometimes with a juvenile rind (~max = 5 cm, ML = 9 cm).

• N o n g r a d e d lapilli and block fall composed of reddish ash coated scoria. This layer is very rich in lithic clasts, mainly consisting of red vesiculated ones and fresh lavas sometimes with a juvenile rind (~rnax = 7 cm, ML = 10 cm).

Nongraded lapilli fall composed of reddish ash coated scoria.

Symmetrically (reverse to normal) graded lapilli and block fall composed of vesiculated black scoria (~3max = 15 cm).

_ • - N o n g r a d e d lapilli fall composed of brown-

reddish coated scoria (~rnax = 3 cm).

~ W e a k l y reverse graded fall composed of black J vesiculated scoria (~max = 4.5).

~ R e v e r s e graded lapilli fall composed of brown ash coated scoria (~max = 1 cm).

~ C o a r s e grained red ash.

data. In particular, there is no indication in the work of Johnston-Lavis of fine ash layers intercalated with the main lapilli layers, in both proximal and distal sections, and the number and description of the various lapilli falls coincides in all details with our stratigraphic sequence. Therefore, our reconstruction of the Mount Somma crest stratigraphy differs significantly from that presented by Bertagnini et al. (1987, 1991) and Santacroce et al. (1993).

The stratigraphic sequence which we have assigned to the 1906 eruption is characterized by the presence of three main lapilli falls and by numerous minor lapilli layers which show lateral continuity inside the dispersal area of the deposits of this eruption. In particular, the transition between the main lapilli falls occurs, in a perfect depositional continuity, through $2 and $3 minor lapilli layers (Fig. 10). The beginning of the eruptive se-

quence is marked by a coarse-grained ash layer that on the Mount Somma crest lies on the erosional unconfor- mity separating the 1906 and 1779 deposits. Deposits of the 1682 and the 1779 eruptions are present on the Mount Somma crest under the 1906 tephra. The 1682 deposit differs from all the other tephra of the post-1631 sequence. The 1682 deposits consist of very vesicular elongated lapilli mixed with denser black lapilli. Dark mica phenocrysts inside 1682 scoriae are easily recognizable in the field. The lithics content is very low. It is important to note that all the eruptive sequences we have identified have lateral continuity and their stratigraphic position are in agreement with their assigned ages (in this case 1682, 1779, and 1906).

Our description of the 1682 lapilli fall agrees perfectly with the description given by Bertagnini et al. (1987, 1991) for their 1906 member "A/ ' In contrast with this

Fig. 11 Outcrop on the Mount Somma crest showing numerous erosional unconformities between tephra of different historical eruptions

stratigraphic reconstruction, Bertagnini et al. (1987, 1991) consider all the lapilli layers between 1631 and 1944 outcropping on the Mount Somma crest as belonging to the 1906 eruption and define as 1906 type section one of the five they described in this site. Three main lapilli layers (A, B, CDE) separated by centimeter-thick coarse ash beds are described by these authors only for the sections on the Mount Somma crest. No trace of these coarse ash centimetric layers separating the A, B, and CDE lapitli are present in any of all other 1906 tephrostratigraphic sections of the entire Somma-Vesuvius area investigated by Bertagnini et al. (1987, 1991). The present work has established that these thin layers of coarse ash, rather than constituting primary fall deposits, actually mark erosional tmconformities (Fig. 2d). The occurrence of numerous erosional unconformities (Fig. 11) and the incomplete knowledge of post-1631 stratigraphy are probably the cause of the hardly explain- able lateral variations (over a distance of few hundred meters along the Mount Somma crest) of thickness and number of lapilli layers assigned to 1906 in the five sections described by Santacroce et al. (1993, p 386).

In conclusion, the reconstruction of the stratigraphy of the entire post-1631 period of the present work indi- cates that the two lowermost main lapilli fall layers (A and B) of the sequence attributed by Bertagnini et al. (1987, 1991) to 1906 actually belong to the 1682 and 1779 eruptions, respectively.

The incorrect stratigraphic reconstruction of the 1906 eruption made by Bertagnini et al. (1987, 1991) had some important implications in the understanding of the working mechanism of Vesuvius for what concerns the entire post-163 t period.

The 1944 eruption

The tephra sequence of the 1944 eruption consists of scoriaceous lapilli fall and hot avalanche deposits (Hazlett et al. 1991). The stratigraphic sequence of the

143

fall deposits in the proximal area has been studied by Fulignati et al. (1996, 1998) who recognized a lithic rich fall on the Vesuvius cone and on the caldera floor east of the crater. Outside the proximal sector the 1944 lapilli fall deposit, which represents the very last activity of Ve- suvius, is often affected by reworking due to human activity. For this reason the thickness of this deposit measured at 27 medium-distal stratigraphic sections all over the dispersal area are considered as minimum.

In the outcrops we observed, the median-distal deposit consists of a lithic-free black lapilli fall bed with reverse to normal grading. The deposit contains aggre- gates of leucite and loose crystals of pyroxene which can attain centimetric size (Table 4). The middle part of the deposit contains abundant centimeter-size loose crystals of olivine probably resulting from the breaking up of ul- tramafic nodules also present in the deposit (Cigolini 1998; Cigolini and Ruffini 1998; Fulignati et al. 1998; Marianelli et al. 1999). On top of the most complete sections, another bed consisting of smaller, less vesiculated, equidimensional gray ash coated scoria has been identified.

Phreatomagmatic activity

Among the tephra deposits of the post-1631 activity of Vesuvius the uppermost lapilli fall layers of some eruptions show physical characteristics which strongly suggest that they are the result of phreatomagmatic volcanism which developed before the end of the eruption.

These characteristics are: (a) high lithic content; (b) ash coating of the clasts; (c) low vesicularity of juvenile fragments; and (d) the presence of fine ash layers with vesiculated texture and accretionary lapilli. Such characteristics are found in the uppermost layers of the 1779, 1794, 1822, and 1906 eruptions. For the 1906 eruption these features had already been interpreted as a result of magma-water interaction (Barberi et al. 1989; Cioni et al. 1992; Mastrolorenzo et al. 1993). The presence of deposits related to a phreatomagmatic phase was also recognized among the products of the 1944 eruption by Fulignati et al. (1996). Therefore, the eruptions of 1779, 1794, 1822, 1906, and 1944 have to be considered the only eruptions with a significant phreatomagmatic phase in the post- 1631 Vesuvius period.

The identification of phreatomagmatic phases inside Vesuvius post-1631 activity has played an important role in the interpretation of its eruptive dynamics. On the basis of historical chronicles, Carta et al. (1981) defined four kinds of behavior at Vesuvius (rest, strombolian cone building, intermediate eruptions, final eruptions), their respective occurrence being controlled by probabi- listic laws. As defined by Carta et al. (i981) the "final" eruption is that which is followed by a period of complete rest, which in the investigated period never exceeds 7 years. Arn6 et al. (1987) introduced the concept of eruptive cycles, subdividing the post-163l period into 18 cycles.

144

Civetta and Santacroce (1992) and Andronico et al. (1996), mainly on the basis of studies of the 1906 eruption, interpreted most of the "final" eruptions of the 18 cycles established by Arnb et al. (1987) as having a violent phreatomagmatic character which involved fluids from shallow hydrothermal systems as suggested (they stated) by the occurrence in the 1872, 1906, and 1944 eruptions of cocoa-colored lapilli containing abundant hydrothermally altered lithics. According to Santacroce et al. (1993) the end of each cycle of activity (the so- called final eruption) should be a violent phreatomagmatic phase. Neither such cycles nor the presence of a phreatomagmatic phase in the 1872 eruption are supported by the present work. In particular, the entire deposit of the 1872 eruption consists of a single lapilli layer with a maximum thickness of 25 cm that is practically lithic- free (0.1 wt.% in Table 4).

It appears therefore that during the post-1631 period only 5 (1779, 1794, 1822, 1906, and 1944) of the 18 eruptions considered as "final" in the cycles of Arnb et al. (1987) had a real phreatomagmatic character. The va- lidity of the subdivision into cycles proposed by Arnb et al. (1987) appears to be questionable now because they do not represent actual cyclic sequences of events. Therefore, the terms "final" and "intermediate" should be assigned only to those eruptions followed by a period of complete rest, or by normal strombolian activity, respectively, as previously defined by Carta et al. ( 1981 ).

Volumes

The volume of the tephra deposits was calculated using the Fierstein and Nathenson (1992) method only for those eruptions for which it was possible to draw a suffi- cient number of isopachs. The calculation was done using the >10-cm isopachs for the 1682, 1723, 1822, and 1906 eruptions, and the >2-cm minimum thickness for

1707 and 1730. For all these eruptions the thickness of the deposit rapidly decreases with distance along a single slope segment.

The thickness of the 1822 deposit extrapolated to the source vent is in good agreement with the values measured at the time of the eruption. In fact, Monticelli and Covelli (1823, see Table 3) report 163 cm on the southeastern rim of the Vesuvius main cone, 130 cm on the eastern, 144 cm on the southern, 85 cm on the western, and 72 cm on the northern rim. These values are fully compatible with the observed southeastern dispersal of the deposit and are in good agreement with the value of approximately 120 cm obtained by extrapolating the curve to source on the lnT vs A u2 diagram. The volumes of tephra deposits of the 1822 and 1906 eruptions do not take into account the fine ashes deposited during the ter- minal phase of these two events. For these two more complex eruptions whose deposits are composed of several units, volumes have been calculated on the basis of dispersal data for each single lapilli layer (Table 5).

The obtained tephra volumes (Table 5) are of the order of 106 m 3 except for the 1822 and 1906 eruptions for which the total volume of tephra is one order of magnitude greater (107 m3). The contribution to the dense rock equivalent (DRE) volume of the abundant lithic fraction present in the upper, thicker layers of these two eruptions (Table 4) is 1.7x107 m 3 for 1906 and 3.4x106 m 3 for 1822. Even if these lithic volumes are subtracted, the DRE volume of these layers remains of the order of 107 m 3, thus indicating that the major magnitude is main- tained for the more complex eruptions not only in their entirety but also layer by layer.

Historical thickness data are generally related to the total 1906 deposits, including the final ash layer. Values /'or the areas enclosed by the 90-, 50-, 30-, 15-, 7-, and 5-cm isophacs of the entire pyroclastic deposit are reported by Imbb et al. (1959; see Table 3). Using these data with the Fierstein and Nathenson (1992) method, a

Table 5 Volumes of post- 1631 lapilli tails. The volume of each layer was calculated by using the plot of the thickness vs square root isopach area (Fierstein and Nathenson 1992). Bulk densities of the different lapilli layers were calculated by pouring the weight- ed total sample into a graduated container and until achieve- ment of a constant volume. DRE (dense rock equivalent) volume was calculated assuming an original magma density of 2.5 g/cm 3. DRE magma volume (of juvenile fraction) was calculated assuming the lithic content of each layer given in Table 4. VEI Volcanic Explo- sive Index (Newhall and Self 1982)

Eruption Lapilli layer Bulk volume Bulk density DRE volume DRE VEI (xlO 6 m -~) (g/cm 3) ( x l O 6 m 3) magma

volume (x 106m 3)

1906 EX l 3.67 0.99 1.45 1.45 2 EX2 6.41 1.09 2.79 2.24 2-3 EX3 49.88 1.36 27.13 9.98 3 Total 71.32

1822 GO 4.32 0.97 1.68 1.63 2 GI,2,3+BSRI,2,3 32.70 1.24 16.22 12.82 3 Total 38.02

1730 1.18 0.86 0.41 0.41 1-2 1723 8.04 1.08 3.47 3.43 2-3 1707 1.29 0.87 0.45 0.45 1-2 1682 5.56 1.02 2.27 2.10 2 1630 Furnas a L1 18.26 b 0.5 3.658 3.658 2-3 1630 Furnas ~ L2 6.038 0.75 1.818 1.818 2 1630 Furnas a L4 6.42 b 0.73 1.87 b 1.87 b 2 1630 Furnas. L5 6.74 b 0.73 1.968 1.968 2

"Data from Cole et al. (1995) b Recalculated from isopach maps drawn by Cole et al. (1995)

145

volume of 101• m 3 was obtained. The same total volume of approximately 100• is given by De Luise (1970; see Table 3) who takes into account also thicknesses <5 cm.

The difference between contemporary values and the volume given in this paper (1.01xl08 vs 7.1• m 3) pos- sibly depends on the contribution of the phreatomagmatic final ashes. The volume of these ashes, on this basis, should represent 30% of the total 1906 tephra volume.

In addition, other historical data (Sabatini 1906; see Table 3) points out the presence of a significant quantity of fine ash in the volcanic plume of the 1906 eruption. According to Sabatini (1906) the thicknesses of this ash deposit (probably related to the last days of the eruption) increased in the distal area (as far as the Adriatic sea coast) and its volume, calculated over a NE sector between 16 and 164 km from the crater, was 2.8x107 m 3.

Arnb et al. (1987) and Civetta and Santacroce (1992) considered the activity within each of the post-1631 cycles as strombolian linked to minor effusive eruptions ("intermediate eruption"), whereas "final" eruptions closing each cycle had a mixed violent explosive and effusive character as well as greater volumes, for both tephra and lavas; however, the volumes calculated in this work (Table 5) indicate that eruptions defined as "intermediate" (1723, 1730) had an important explosive component with volumes sometimes greater than those of "final" eruptions (1682, 1707).

Column height

Some information on eruptive column heights is in the historical documents describing the eruptions (Table 6). The eruptive column heights reported in Table 7 were

moreover calculated with the method of Wilson and Walker (1987). Average mass discharge rate (MDR) was calculated using the information given by historical chronicles concerning the duration of the various eruptive pulses and the mass calculated from the deposits.

Values of the various column heights, ob ta ined by means of the theoretical relationship between Ht (column height) and bt (thickness half distance) as defined by Sparks et al. (1992), were also calculated. The different methods utilized to calculate the height of an eruptive column in case of non-plinian eruptions lead to different values, as listed in Table 7. The values obtained with the method of Sparks et al. (1992) appear to be sub- stantially uniform, a fact that is in contrast to the dispersal differences observed between the eruptions.

The correlation between the dispersal area of the deposit of each eruption and the heights of their respective eruptive columns appears to be more consistent when applying the method of Wilson and Walker (1987) which obtains the intensity of an eruption from the physical characteristics of its deposit. In this case in fact it appears that: (a) the 1822 and 1906 eruptions are more significant than the others, having maximum column heights of the order of 10 km; (b) these two same eruptions have column heights of the initial purely magmatic phases in the order of 5 km; and (c) the 1682, 1707, and 1723 eruptions, with similar volume and deposit characteristics both with each other and with the initial phases of the 1822 and 1906 eruptions, have maximum column heights of 2-3 km.

In this context, the 1730 eruptive column was anoma- lously high (5.8 km) due to the duration of the eruption which lasted only 45 rain as described in historical chronicles.

Table 6 Column heights (kilometers above the crater) of post-1631 eruptions from contemporary sources (Table 3)

Ernption/layer Day Hour Column height (m) Data source

1779 OTV2 8 August - 1779 OTV2 8 August --7 p.m. 1779 OTV2 8 August =7 p.m. 1779 ash 9 August 4 p.m. 1822 GO 22 October 1 a.m. to dawn 1822 ash 22 October 1 p.m. 1906 EX 1 7 April Evening 1906 EX 1 7 April Evening 1906 EXl 7 April 8-10:45 p.m. 1906 EXI 7 April 8-10:45 p.m. 1906 EX2 7 April 11 p.m. 1906 EX3+upper 8 April Morning

lapilli layers 1906 EX3 8 April 0.45-3 a.m. 1906 ash 8 April - 1906 ash 8 April - 1906 ash 8 April - 1906 ash 8 April - 1906 ash 8 April 3 p.m. 1906 ash 13 April - 1906 ash 14 April - 1906 ash 17 April - 1906 ash 20 April -

=2700 =2700

1800 4500

600-700 >3000

900 800-1000

2000 1000-2000

> 1000 4O00

>4300 =6500 4000-6000 (max. 13,200) 4000-5000

>7000 13,000 3100 1400 900 600

Torcia, 1779 Hamilton, 1779 Anonymous (Abate Galiani), 1780 Anonymous (Abate Galiani), 1780 Monticelli and Covelli, 1823 Monticelli and Covelli, 1823 De Lorenzo, 1906 Matteucci et al, 1906 Lacroix, 1906 Mercalli, 1906 ("L'eruzione Vesuviana...") Bassani and Galdieri, 1906 (Toniolo, 1906) Bassani and Galdieri, 1906 (Toniolo, 1906)

Baratta, 1906 De Lorenzo, 1906 Matteucci et al, 1906 Mercalli, 1906 ("La grande eruzione...") Johnston-Lavis, 1909 Perret, 1924 De Luise, 1907 De Luise, 1907 De Luise, 1907 De Luise, 1907

146

Table 7 Column heights of the post- 1631 eruptions from field data. The duration of each phase is deduced by the analysis of historical chronicles. Mass discharge rate (MDR) at the vent calculated from the bulk density and bulk volume of the deposit. (bt) parame-

ter is calculated following Pyle (1989) by using the plot of In thickness vs square root isopach area plots. D=138.7 bt 2 (Hough- ton et al. 2000)

Eruption Lapilli layer Duration MDR bt (km) (h) a (• kg/s)

D (kin 2) Wilson and Walker Sparks et al. (1987) (1992) Column height (km) Column height (km)

1906 EX l 3 0.34 0.99 EX2 1.5 1.29 0.87 EX3 3.5 5.38 1.97

1822 GO 5 0.23 0.43 G1,2,3+ BSR1,2,3 3 3.75 1.53

1730 0.75 0.38 0.82 1723 106 0.02 0.74 1707 53 0.006 0.94 1682 192 0.008 0.58 1630 Furnas b L2 2.77 3-4 c 0.50 d 1630 Furnas b L5 1.12 =2 c 0.46 a

135.94 5.7 = l 3 104.98 7.9 =12 538.28 11.4 =15.5

25.65 5.2 -10 324.68 10.4 = 15 93.26 5.9 =12 75.95 2.8 ~-11

122.55 2.1 =12.5 46.66 2.2 =10 34.67 d 10.6 -11.5 a 29.35 d 8.9 =ll d

a From historical chronicles b Data from Cole et al. (1995) c Calculatios of mass flux erupted from the vent using the models of Wilson and Walker (1987) d Recalculated from isopach maps drawn in Cole et al. (1995)

In Tables 5 and 7 we include for comparison the data related to the 1630 eruption of Furnas volcano (Cole et al. 1995, 1999) that produced deposits similar to the mi- nor Vesuvius post-1631 ones. The volumes in Table 5 have been recalculated from the isopachs of the afore- mentioned authors and appear to be of the same order of magnitude of the smaller post-163t eruptions of Ve- suvius, therefore one order of magnitude lower than that calculated by Cole et al. (1995, 1999). The intensity of the 1630 eruption of Furnas was obtained by Cole et al. (1995, 1999) by applying the model of Wilson and Walker (1987) to lithic isopleth data. In this way they obtained a column height value greater than the volumetrically similar Vesuvius post-1631 eruptions.

Systematically lower values are observed for historical data as compared with those calculated with the method of Wilson and Walker (1987; Tables 6, 7). It is noteworthy, however, that their method, as well as other parameterizations of modern physical volcanology, was calibrated on true plinian eruptions such as those of Mount St. Helens in 1980, Fogo in 1563, St. Maria in 1902, Askja in 1875, etc. On the contrary, in the case of low-magnitude eruptions, such as those of the post-1631 period at Vesuvius, the physical laws which control the distribution of clasts are not necessarily the same.

Eruption styles of the post-1631 period

There is a certain ambiguity in the volcanology literature as to which physical parameters should be used to define classes for the low-energy explosive events (non-plinian eruptions).

The term "strombolian", as used in the literature, frequently refers to those eruptions where the ejected pyroclasts (bombs and lapilli) rarely reaches a few hundred meters above the vent. The deposits of such eruptions are

generally described as scoria cone forming. They thin out rapidly away from the vent and only ash may extend beyond the cone for several kilometers downwind, gen- erating an abrupt topographic break between the steep- sided cone and the flat sheet of ash fall (Self 1976; Francis 1996; Houghton et al. 2000).

All eruptions studied in the present work are characterized by lapilli and ash fall deposits resulting from eruptive columns some kilometers high and without the construction of major scoria cones. For these reasons they cannot be classified as "normal strombolian eruptions."

In the classification terminology of Walker (1973) they fall between the "violent strombolian" and the "subplinian" eruptions, with a D parameter between 5 and 500. However, the distinction between these two catego- ries is not clearly stated in the literature. Also the limit between "subplinian" and "plinian" appears to be some- what unclear (Cioni et al. 2000).

Amos et al. (1981) studied the 1065 eruption of Sun- set Crater in Arizona, which produced a total VDRE=0.30 km 3 of scoria fall composed of several indi- vidual fall units with VDR E up to 4• m 3. Volume and dispersal data indicate that the magnitude of this eruption was much larger than a normal strombolian event. In addition, inferences from the study of those deposits suggest that the eruption was characterized by a sustained convective column much higher than a typical strombolian one. According to Francis (1996) the presence of a sustained eruptive column is a parameter that categorizes that Sunset Crater eruption as subplinian.

This is also the case for the 1986 eruption of Oshima (Izu, Japan) which, with a fall volume of 1.6• m 3, was considered as subplinian. During this eruption fire fountains reached 1.6 km, whereas the convective part of the eruptive column reached an altitude of 16 km (Francis 1996).

Table 8 Bulk volumes of eruptions classified as subplinian in the literature

147

Volcano Locality Eruption Bulk volume Source data (• 106 m 3)

Mono Craters USA Bed 1 38 Sieh and Bursik (1986) Bed 2 18 Sieh and Bursik (1986) Bed 3-6 65 Sieh and Bursik (1986) Bed 7 12 Sieh and Bursik (1986)

Pico Alto Terceira, B 330 Self (1976) Azores E 220 Self (1976)

C 40 Self (1976) I 30 Self (1976)

Santa Barbara Terceira, D 90 Self(1976) Azores G 90 Self (1976)

H 90 Self (1976) A 80 Self (1976) F 30 Self (1976)

Hekla Iceland 1970 70 Thorarinsson and Sigvaldason ( 1971) St. Helens U.S.A. 18 May 1980 189 Scandone and Malone (1985)

25 May 1980 16.4 Scandone and Malone (1985) 12 June 1980 20.0 Scandone and Malone (1985) 22 July 1980 3.0 Scandone and Malone (1985) 7 August 1980 1.5 Scandone and Malone (1985)

16 October 1980 1.0 Scandone and Malone (1985)

On the basis of the great altitude of the ash column of the 1872 Vesuvian eruption, depicted in contemporary paintings, Francis (1996) also classified this eruption as "subplinian." However, the lapilli fall deposit investigated during the present work is not characterized by a high thickness over a wide area. The column height considered by Francis (1996) was more likely connected to the final vapor-rich phase of the eruption rather than to the actual height of the column which produced the lapilli fall deposit.

The subplinian deposits of Terceira (Azores) have smaller volumes (Table 8) than plinian eruptions and are considered by Self (1976) to be scaled-down versions of the plinian type. The gas blast producing a subplinian deposit is interpreted as being less continuous than that which produces a plinian deposit because of the evident internal stratification which suggests pauses during the eruption. The smaller dispersal of subplinian deposits may result, according to Self (1976), from the fact that a characteristic fi- nite time is required for a fully developed eruption column to form. If the duration Of each gas blast phase is not much greater than this characteristic time, then, on average, parti- cles will be released while the column is still growing, thus falling from lower altitudes than those they would have reached in a steady convective colutrm.

Data reported in Table 8 indicate that eruptions considered in the literature to be subplinian have a bulk volume always greater than 106 m 3 and generally not lower than 107 m 3.

A general classification of plinian and strombolian eruptions has been made by NewalI and Self (1982), using the volcanic explosivity index (VEI) to define the magnitude of an eruption on the basis of gross variation intervals of volume and column height.

Finally, Houghton et al. (2000) make a distinction between plinian, subplinian, and hawaiian/strombolian

eruptions on the basis of the area contoured by each isopach and related thickness of the deposit, taking into consideration also the few data available in the literature for non-plinian eruptions. In particular, to define "subplinian" eruptions these authors used information from Hekla (Iceland) in 1970, two ancient eruptions of Pico Alto (Terceira, Azores), and Paricutin (Mexico) in 1944-1945.

In the same kind of diagram (Fig. 12) we report data for the post-1631 eruptions and, for comparison, the two plinian phases known as pomici grigie and pomici blan- che of the 79 A.D. eruption of Vesuvius (Sigurdsson et al. 1985) as an example of a true plinian eruption. There is a volume difference between the 1822 and 1906 eruptions and all other eruptions of the post- 163 l period. All the curves referring to the post-1631 eruptions show a trend of the "hawaiian" and "strombolian" kind with the exception of the topmost members of the 1822 and 1906 eruptions which occupy, in the Houghton et al. (2000) classification, an intermed~.ate position between "hawaiian/strombolian" and "subplinian" eruptions. These two eruptions were characterized by the development of a sustained convective column phase with a height of the order of 10 km, volumes greater than 107 m 3, bt>l.5 and intensity MDR values not lower than 106 kg/s.

In order to try to move toward a common terminology, criteria for defining eruption style have to be based on properties directly connected with the deposit. In light of all the mentioned studies and looking at the characteristics of the deposits and their dispersal, it seems that one of the most relevant parameters in discriminating between "subplinian," "plinian," and "violent strombolian" eruptions is the intensity value (MDR). The MDR is lower for subplinian events than for plinian, and this is the main factor controlling eruption column heights and dispersal areas. Bower and Woods (1996) give intensity

148

Fig. 12 Plot of In T vs A I/2 for the post-1631 deposits. Data of white and gray Ptinian pumices erupted in 79 A.D. are also shown for comparison

10000

E 1000

E

C

. c 100

10

lg06 EXl

1906 EX2

1906 EX3

1822 GO

- ~ 1822 G1,2,3+BSR1,2,3

--~ 1730 A.D.

1723

1707

�9 1682

., 79 A.D. gray fallout

7g A.D. white fallout

\ ,

I

10 I I I I

20 30 40 50

(area)lm(km)

values for subplinian eruptions between l06 and 107 kg/s. These values are applicable only to those of 1822 and 1906 which for this reason are the only two post-1631 eruptions that can be classified as subplinian. All other post-1631 eruptions with an explosive component can be classified as violent strombolian, due to the presence of lapilli fall generated by eruptive columns just a few kilometers high, a volume of the lapilti fall deposits not exceeding 106 m 3, MDR <106 kg/s, and bt<l.5. It is important that values of timing in the MDR calculation came from direct observations to avoid MDR values derived from numerical models and limiting as- sumptions. This is the major limitation in assuming the MDR value as a classification criterion. In fact, directly measured timing is disposable only for a restricted number of events which occurred in the past two millennia. Alternatively, the most suitable criterion results in the use of physical parameters like bt, connected with the tephra dispersal. Nevertheless, in that case we have to take into account that clast dispersal is influenced by the presence of wind, which is a factor external to the volcanic system.

It is noteworthy that the 1779 and 1794 eruptions show similar stratigraphic complexity as those of 1822 and 1906. This might suggest that they also had a "subplinian" kind of character. However, it has not been possible to establish for them the values of the same physical parameters as for 1822 and 1906 eruptions and con- sequently define with the same certainty the category to which they belong. This was due to the insufficient number of stratigraphic sections identified in the field.

Summary and conclusion

The complete reconstruction, over all the Somma-Ve- suvius area, of the tephrostratigraphy of the post-1631 period of activity allows us to draw the following main conclusions:

1. The explosive component of the numerous eruptions of this period has always been considered as minor, except for those of 1906 and 1944. However, in addition to the deposits of 1906 and 1944, we have identified 14 more tephra episodes, testifying to an important explosive component. These 16 tephra deposits are the only ones preserved in the area outside the Mount Somma caldera.

2. The age of those tephra deposits has been established on the basis of their stratigraphic position in relation to historically dated lava flows and substantiated by the information in historical chronicles.

3. The stratigraphic reconstruction of the 1906 eruption made by Bertagnini et al. (1987, 1991) is incorrect in that it likely includes older beds of the post-1631 activity.

4. A phreatomagmatic character has been proven only for 5 of the 18 eruptions considered as "final" in the literature. Therefore, the subdivision into cycles proposed by Arn6 et al. (1987) is questionable; indeed, it does not represent actual cyclic sequences of events. The terms "final" and "intermediate" should be assigned only when closely corresponding to the defini- tions of Carta et al. (1981).

5. The physical parameters of the tephra deposits investigated have allowed us to obtain reliable data on the volumes, column heights, thickness half distance, and

MDRs of some of the considered eruptions. These data have been used to classify those eruptions with an important explosive component. Three main styles have been identified: (a) periods of violent strombolian activity; (b) violent strombolian eruptions; and (c) subplinian eruptions.

6. During the first century of activity after 1631, there were two periods of violent strombolian activity (1682-1707 and 1707-1718/19), preceded and followed by a series of violent strombolian eruptions (1660, 1682, 1707, 1723, 1730, 1790, 1872). Between 1730 and 1779 a relevant change in the eruptive style of Vesuvius was marked by a decrease in the number of eruptions with an explosive component and by a concomitant increase in intensity and magnitude, from violent strombolian toward subplinian (such as 1822 and 1906).

Acknowledgements The authors thank the Directorate of the Os- servatorio Vesuviano and the personnel of the Historic Centre of the Observatory for the hospitality given at the Eremo del Salva- tore during the entire period of field work. We are grateful to A. Melilli for the help given during both the field work and the analysis of historical chronicles, and to G.M. Di Paola for a critical reading of the manuscript. The paper benefitted from the critical reviews by B. Houghton and G. MacDonald. This study was supported by funding from CNR-IGGI and by CNR-National Group of Volcanology.

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Violent strombolian and subplinian eruptions at Vesuvius during post-1631 activity

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